252 D. F. JONES. 



gametes and zygotes. Davis (1915-' 17) has called attention to 

 the great amount of pollen and ovule abortion in these plants 

 and to the low germination of the seeds which are produced 

 and to the results which are obtained when a more complete 

 germination is secured. 



It is not always easy to distinguish between differential destruc- 

 tion of zygotes immediately after fertilization and selective 

 fertilization. It is still more difficult in the case of abortion of 

 germ cells. In fact it is only a matter of degree between selective 

 elimination of gametes and selective fertilizing ability- From 

 germ cells which are unable to complete development on account 

 of the particular inheritances they carry, to gametes which 

 appear normal but are unable to function perfectly under any 

 circumstances, it is only a step. However, no instance of the 

 latter condition is positively known and the cases of gametic 

 abortion clearly due to the factors which the gametophytes 

 carry are rare. 



Although results still remain to be cleared up which indicate a 

 selective action of some kind (Kempton, 1919) the general con- 

 clusion holds, which is, that the pollen, although gametophytic 

 itself, has the function of the sporophyte which produces it. 

 This is supported by the well-known cases where pollen color 

 and shape are all of the type of the maternal parent irrespective 

 of the factors for pollen characters which they carry. Many 

 instances are known where gametes, containing lethal factors 

 which stop development immediately after fertilization, are able 

 to function fully as well as others permitting normal growth. 

 East and Park (1917) have shown that compatibility in Nicotiana 

 is alike for all the gametes of any one individual. Different 

 pollen grains may carry factors which will determine sterility 

 or fertility of the future zygotes but all function alike in fertiliza- 

 tion according to the sporophyte from which they come. Follow- 

 ing up these results East (1919) shows that in the frequency dis- 

 tributions of pollen tube lengths there is no significant difference 

 between pollen from plants greatly unlike in the degree of 

 heterozogosity. In other words gametes carrying markedly di- 

 verse germinal complexes are no more variable than those all of 

 like constitution in ability to send pollen tubes down the styles 



